1. Field of the Invention
This invention relates generally to nuclear medical imaging, and more specifically, to improvement in calibration of multiple detector head positioning in SPECT imaging.
2. Background and Prior Art
In nuclear imaging, a patient is injected with or swallows a radioactive isotope which has an affinity for a particular organ, structure or tissue of the body.
Gamma rays are then emitted from the body part of interest, are collimated by a collimator so that only gamma photons traveling in a direction perpendicular to the surface of a detector head are allowed to impinge on the detector head, and are detected by a gamma camera apparatus including the detector head, which forms an image of the organ based on the detected concentration and distribution of the radioactive isotope within the body part of interest. Nuclear images may be obtained using single photon emission (either planar or Single Photon Emission Computed Tomography (SPECT)) and Position Emission Tomography (PET). Planar imaging essentially compresses a three-dimensional radiation field onto a two-dimensional image plane, while SPECT and PET produce multiple image “slices,” each representing a different plane in a three-dimensional region, such that when the slices are considered collectively, a three-dimensional image of the region may be studied.
Among the various factors that cause SPECT image distortion, detector head deflection and alignment issues are among the largest sources of error. Siemens Medical Solutions USA, Inc. recently introduced an imaging platform that provides a versatile mechanical system that allows the detectors to scan at optimal positions for various studies, such as cardiology studies. However, the versatility of the mechanical platform amplifies the challenges involved in accurate detector head position calibration because it introduces the capability for new detector positions and accordingly the conventional projection geometry assumptions used in image reconstruction are no longer applicable, leading to potential for image blurring and distortion.
While one way to reduce such problems is mechanical measurement of the detector heads, such a procedure is costly and still cannot eliminate the errors caused by the electronics and other system factors.
It is known to calibrate SPECT detector heads by using a Multi Head Registration (MHR) phantom, which incorporates five radioactive point sources arranged on a plane. The planar arrangement limits MHR to two-dimensional analyses, such that no more than two of the five possible positional parameters can be corrected. Experiments have shown, however, that at some positions all five position-parameters can have large disparities from frame to frame; consequently correction of the x coordinate with one overall value and the y coordinate for each frame is insufficient and can result in degradation of image quality. The two-dimensional limitation of the MHR phantom calibration is a result of the absolute spatial coordinates of the phantom being unknown. Instead, an iterative algorithm is used to estimate the phantom coordinates, which are determined on a planar configuration as such is easier to converge.
Siemens Medical Solutions has developed a new calibration method for a hybrid SPECT/CT imaging system that by utilizing CT data to establish a global spatial coordinate system of a common test phantom. See copending U.S. patent application Ser. No. 11/091,253, incorporated herein by reference in its entirety.
In such method, the common test phantom is used to obtain a set of point source nuclear images. Gaussian peaks are fitted to each point source for each frame of projection data, to obtain a set of projected centroid data. A pair of cones is then generated by using the geometric correspondence between the three-dimensional centroids and their projections on the two dimensional image plane. Among the two bundles of intersecting lines from the generated pair of cones, one pair of lines will form an angle that is equal to the angle between the projections. This pair of lines is then determined, which yields the normal of the detector plane. This normal is then used for solving five correction parameters for each view angle.
The correction parameters are used to offset displacements of the detector head during rotation around a subject, as well to correct misalignments in the two-dimensional image plane of the detector head with respect to the fixed coordinate system of the CT imaging modality of the hybrid system.
Thus, according to the hybrid SPECT/CT calibration method, an absolute coordinate system is established in the center of the CT Field of View (FOV). When the phantom is moved to the SPECT FOV, a coordinate transformation matrix can be obtained by recording the displacement of the patient bed (on which the phantom is placed). The transformation matrix is then applied to derive the absolute coordinates of the phantom in the SPECT FOV. Once the absolute coordinates of the phantom are known, point source projections of the phantom can be calculated regardless of the geometrical complexity of the phantom.
Accordingly, there remains a need in the art for improvement in correction of projection image positional errors in order to improve the accuracy and quality of such images when used in SPECT image reconstruction. In particular, it would be desirable to be able to establish an absolute coordinate system for a calibration phantom in a SPECT FOV for a SPECT imaging system, without performing an actual CT scan.